1. Field of the Invention
The present invention relates to apparatus and methods for covering devices, such as required when delivering an expandable device, such as an intraluminal stent or graft.
2. Description of Related Art
Stent and stent-graft usage has gained widespread acceptance by radiologists, cardiologists, and surgeons. These devices are being utilized to radially support a variety of tubular passages in the body, including arteries, veins, airways, gastro-intestinal tracts, and biliary tracts. The preferred method of placing these devices has been to use specialized delivery systems to precisely place and deploy a device at the site to be treated. These delivery systems allow the practitioner to minimize the trauma and technical difficulties associated with device placements. Attributes of delivery systems include: low profile; ability to pass through introducer sheaths; ability to negotiate tortuous vasculature, smoothly and atraumatically; protection of constrained devices; and ability to accurately position and deploy the device.
Traditionally, stents or stent-grafts have been designed either to plastically deform (e.g., "balloon expandable" stents) or to elastically recover (e.g., "self expandable" stents) from a collapsed, introduced diameter to an expanded, functional diameter. Stents that are typically designed to elastically recover are manufactured at their functional diameter, and then radially compressed to be mounted on a delivery catheter. These devices must often be constrained in this compressed state for a prolonged period of time. Additionally, there must be a mechanism to release this restraint remotely and allow the device to elastically recover to its functional diameter when properly positioned.
A number of techniques are practiced to constrain elastically compressed stents and allow the restraint to be removed from a remote site. One technique involves placing the stent in the annular space between two concentric catheter tubes. The inner tube facilitates passage of a guide wire through its inner diameter, and the stent or stent graft is elastically compressed on its outer diameter. The outer catheter tube or sheath then is placed over the compressed device, effectively capturing the compressed device. When it is desired to have the stent recover to its functional diameter, the other tube is pulled back relative to the inner tube, and the device elastically recovers. This deployment can be activated remotely (for example, at the hub end of a catheter) by longitudinally displacing the tubes relative to each other.
A variance of this concept is to have another concentric tube, located concentrically between the outside and inside tube. By moving this third tube relative to the other tubes, the elastically constrained device can be pushed from the catheter, allowing it to elastically recover to its functional diameter. The inner tube, through whose lumen a guide wire passes, could be removed for a further variation in this design. In the modified design, the guide wire would pass in the lumen of the pushing tube and through an unprotected lumen of the collapsed endoprosthesis.
Another possible technique that can be employed uses a suture that is stitched to the stent or stent graft in its collapsed, elastically constrained diameter. In one embodiment of this technique, a "chain" stitch of removable suture is made through the metal struts of a collapsed stent. One end of the stitch can then be pulled, from a remote location, releasing the stent to elastically recover.
Another technique involves encasing the collapsed endoprosthesis in a thin-walled wall casing that is held in a tubular configuration by a chain stitch of removable thread (e.g., a suture) applied to a longitudinal seam. When the stitch is removed, the seam is opened as the stent elastically recovers to its operational diameter. This release mechanism leaves the thin wall casing captured between the device and the tube in which it was deployed.
The devices employed for constraining elastically deformed stent or stent grafts and remotely deploying them have a number of problems. One desired feature of an undeployed stent is that it be flexible on the catheter. This allows the catheter to be easily manipulated through the path it must negotiate from its entry site to the site where the device is to be deployed. When concentric catheter tubes are employed, this construction usually creates large cross-sectional dimensions and is stiff, making navigation through tortuous vascular segments difficult. Also, the need to manipulate multiple tubular components can make accurate placement of the stent or stent graft difficult. Another typical problem is that large forces are often required to retract the sheath or to push out the stent.
There are many other desirable features for a stent or stent-graft delivery system. For instance, it is very beneficial for the exterior surfaces of the collapsed endoprosthesis to be smooth and, therefore, more atraumatic to host vasculature. Additionally, it is desirable for the system to work with any elastically recoverable stent. Further, it is desirable for the delivery system to have sufficient strength to radially constrain the device during its normal shelf life without "creep" dilatation.
For delivery systems that include a removable suture through the stent, the systems effectiveness in restraining the device depends greatly on the endoprosthesis design. The struts of the endoprosthesis can be exposed in its collapsed diameter, and this can potentially cause trauma during navigation to the treatment site. Additionally, exposed struts may also cause difficulties with deployment (for instance, entanglement of deployment suture or struts).
A disadvantage of the thin-walled casing devices is that the encasing sleeve is left in-vivo after the endoprosthesis has been deployed, which may inhibit healing or endothelialization of the luminal surface and cause flow stream disruption. Also, these devices may add significant profile to the catheter, and are detrimental to the catheter's flexibility. Finally, another problem with current delivery systems that use single removable threads to facilitate deployment of the endoprosthesis is that these cords must be designed to be strong in tension so that they do not break during the removal process. This requirement usually dictates that larger diameter threads must be used than are needed to hold the endoprosthesis in its elastically collapsed configuration to avoid tensile failure during deployment.
Some of these deficiencies are addressed in U.S. Pat. No. 4,878,906 to Lindemann et al. and U.S. Pat. No. 5,405,378 to Strecker. Both of these patents employ a one or more contiguous removable thread around an expandable prosthetic that can be remotely removed through a catheter tube or the like. While these methods of prosthetic deployment may offer some improvement over other deployment methods, the open-structure nature of these constraints are believed to provide only limited and localized resistance to the force exerted by a self-expanding prosthesis. Other possible problems with these devices include: uneven distribution of constraining force radially and along the length of the prosthesis; high stresses on a single deployment suture that may lead to breakage risks during deployment; inadequate coverage of the outside of the prosthesis--possibly leaving a rough exposed surface; and undesirable back-and-forth movement of the constraining/deployment filament over the exterior surface of the device during deployment, which may lead to potential entanglement or embolism formation. This is sometimes referred to as a "windshield wiper" effect.
Accordingly, it is a primary purpose of the present invention to provide an improved apparatus and method of deploying a self-expanding device, such as an endoluminal stent or the like.
It is a further purpose of the present invention to provide an apparatus and method for deploying a self-expanding device that provides excellent constraint and coverage of the device in a non-deployed state and ease of deployment of the device once it has been properly positioned.
These and other purposes of the present invention will become evident from review of the following specification.